ZHOUCHENG, China -- Mention potential Chinese threats to America, and people think of spyplane incidents or conflict over Taiwan, not pigs, ducks, and farmers like Yang Jing Huau.

To Yang and thousands of other villagers in China, livestock are bank accounts on four feet. These investments measure a rural family's wealth and get tender loving care.

That's why new litters of squealing porkers often spend cold nights inside the farmhouse, sleeping near family members. In some areas, livestock occupy the first floor of the family house, and people the second.

"My ancestors have done so for centuries," Yang explained on a bright spring day in this village near Lijiang in China's Yunnan Province. "We have pigs, ducks, and chickens."

All around southern China, people have the same unusually close contact with animals -- on tiny farms, in crowded village markets, and at sprawling big city bazaars that sell live birds and pigs directly to consumers.

Experts on infectious disease believe this close contact has a deadly impact in the United States and around the world.

"This part of the world is the epicenter for influenza epidemics," said Dr. Kennedy Shortridge, an international authority on flu viruses at the University of Hong Kong. "It's the place where epidemics are most likely to originate. The ingredients are here -- ducks, pigs, people -- in close contact."

Ducks are the reservoir for bird flu viruses. Duck droppings litter the soil on hundreds of thousands of small farms throughout China. Pigs constantly snort in the soil looking for food and inhale avian flu viruses from duck feces. Then pigs act as a flu "mixing bowl," combining avian viruses with the human flu viruses they already harbor from contact with people.

Pigs' sleight of hand

Pigs shuffle the genes of human and bird flu viruses like two decks of playing cards. It creates the new strains of influenza that march around the globe each year like an invading army.

The shuffle causes "genetic drift," a slight change in the flu virus's ability sneak past humans' immune defenses. The result is your typical flu season. In the United States that means hundreds of thousands of people stricken and 20,000 deaths.

Periodically, however, humanity gets dealt a really bad hand. Major genetic shifts can occur, leading to the sudden emergence of strains to which most humans have no immunity.

During the last 250 years, a dozen or more pandemics -- worldwide epidemics -- have swept the globe. The "Spanish" flu epidemic of 1918-19 was by far the worst. It sickened 200 million people and killed 20 million, including 500,000 in the United States.

Like other experts, Shortridge believes the 1918-19 pandemic was misnamed because it did not originate in Spain. Rather, he thinks it started with human-pig contact in China and traveled to Europe with Chinese laborers who dug trenches for World War I battlefields.

The origins of other terrible epidemics -- Asian flu in 1957, Hong Kong flu in 1968, and "Russian" flu in 1977 -- also have been traced to traditional Chinese agricultural practices.

New viruses can and do emerge in other countries. "Anywhere in the world where humans, birds, and pigs live in proximity would be a fertile breeding ground for pandemic viruses," said Shortridge.

Ancestral virus

Flu has stalked humanity for thousands of years. Hippocrates wrote a clear description of influenza in 421 B. C. -- including the fever, shaking chills, muscle aches, headache, dry cough, and other symptoms that can put victims in bed for a week or more.

Scientists isolated the first human flu virus in 1933, and named it Type A. It is one of three families, called A, B, and C. The A family causes the great epidemics and the B family causes more localized outbreaks. Type C viruses cause little disease in humans.

The great ancestor of this annual wave of misery and death is a bird influenza virus that emerged in the ancient past. It evolved into five lineages specialized to infect different animals: an ancient horse virus not observed in more than 15 years; a modern equine virus; an aquatic bird virus; a swine virus; and a human virus.

"The ancestral viruses that caused the Spanish flu in 1918, as well as the viruses that provided gene segments for the Asian/1957 and Hong Kong/1968 pandemics, are still circulating in wild birds," said Dr. Robert G. Webster, a flu expert at St. Jude Children's Research Hospital in Memphis.

What's the possible threat to people?

That's where Yang's porkers enter the picture.

Duck, chicken, and other avian flu viruses can't infect people, according to the traditional scientific view. Human cells lack the right docking port where avian viruses can attach and inject their genetic material. That transfer allows a virus to infect a cell, commandeer the cell's genetic machinery, and start making copies of itself. The newly produced viruses then infect other cells, and the unfortunate person takes sick and spreads the virus to others in coughs and sneezes.

Pig cells, however, do have the docking ports for bird flu viruses. They also have docking ports for human influenza viruses. Once inside the pig, avian viruses reproduce and make billions of copies of the original virus. At the same time, human influenza viruses, which pigs catch from people like Yang and his family, also are reproducing inside pig cells.

Imaging shuffling two decks of cards -- one red, one blue. As human and avian viruses reproduce inside pigs, their genes get shuffled and mixed. If a new virus is only a little different from existing flu strains, many people will have some immunity from past infections. The result usually is a run-of-the-mill flu season. If the virus is totally different, one to which people have little or no immunity, brace for a possible pandemic.

A new wrinkle

This traditional view of flu took on a new wrinkle in a 1997 episode that set off alarm bells for another flu pandemic.

It began in May when a 3-year-old boy in Hong Kong died from a strange respiratory illness. Scientists isolated a virus but could not match it with any known strain.

Shortridge solved the mystery. It was an avian flu virus that apparently could infect humans directly, without first passing through the pig mixing bowl.

The virus, known as Type A H5N1, turned out to be the same microbe that had killed thousands of chickens on farms around Hong Kong earlier that year. Although it caused only 18 confirmed human cases of flu and six deaths, H5N1 set off alarms among flu experts.

Each new strain of flu virus usually bears some resemblance to its predecessors. Because of that similarity, most people's immune systems are primed to provide some defense, no matter what form of virus circulates.

H5N1, however, was unlike any previous flu virus, meaning that humans would have little natural defense against infection.

Shortridge, for one, says such viruses -- although capable of causing severe disease -- may be very difficult to transmit from one person to another. He suspects H5N1 was not the first avian virus transmitted directly to people. Rather, avian viruses may have been infecting handfuls of people, and then dying out, for centuries.

Detection of H5N1 led to swift measures to control the virus before it could get into the pig mixing bowl and mutate into an easily-transmitted form. Public health officials in Hong Kong ordered the mass slaughter of domestic ducks and chickens, closed live bird markets, and instituted other measures.

Changing Chinese agriculture

Shortridge believes flu pandemics will continue so long as farmers like Yang Jing Huau continue to live in close quarters with ducks and pigs. "Changing that situation would be difficult," he said. "Remember that one out of every seven people in the world -- some 900 million people -- live on farms in China where pigs and ducks are common."

But agricultural practices are changing on their own as China modernizes and adopts modern approaches like industrial pig and duck farming, in which big farms, run by few people, raise huge numbers of animals.

Also needed are changes in aquaculture, or fish farming, which has been spreading so fast in Asia and other areas that it often is termed "the blue revolution." Increasingly, small-scale farmers supplement their incomes by raising fish in ponds. They use fresh pig and duck manure to fertilize the ponds to encourage growth of plants for fish to feed on. Wild ducks also land in the ponds.

Scientists know avian flu viruses in wild birds spread globally during north-south migrations. Ducks defecate as they paddle through fish ponds and release flu viruses. Pigs, domestic ducks, and other animals pick up the viruses by drinking the water, or wallowing in it.

Christopher Scholtissek and Ernest Naylor in a 1998 study argued that use of fecal material may transform fish farms into flu factories, where pig and avian flu viruses mix and spread.

The Block News Alliance consists of the Post-Gazette and The Blade of Toledo, Ohio. Michael Woods is the Blade's science editor.